Beam delivery system for laser dark-field illumination in a catadioptric optical system

ABSTRACT

A method and apparatus for inspecting a specimen are provided. The apparatus comprises a primary illumination source, a catadioptric objective exhibiting central obscuration that directs light energy received from the primary illumination source at a substantially normal angle toward the specimen, and an optical device, such as a prism or reflective surface, positioned within the central obscuration resulting from the catadioptric objective for receiving further illumination from a secondary illumination source and diverting the further illumination to the specimen. The method comprises illuminating a surface of the specimen at a variety of angles using a primary illumination source, illuminating the surface using a secondary illumination source, the illuminating by the secondary illumination source occurring at a substantially normal angle of incidence; and imaging all reflected, scattered, and diffracted light energy received from the surface onto a detector.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of optical imaging,and more specifically to catadioptric optical systems used forbright-field and dark-field optical inspection applications.

2. Description of the Related Art

Many optical and electronic systems exist to inspect surface featuresfor defects such as those on a partially fabricated integrated circuitor a reticle. Defects may take the form of particles randomly localizedon the surface, scratches, process variations such as under etching, andso forth. Such techniques and apparatus are well known in the art andare embodied in various commercial products such as many of thoseavailable from KLA-Tencor Corporation of San Jose, Calif.

Several different imaging modes exist for optical inspection. Theseinclude bright-field, and a variety of dark-field imaging modes. Each ofthese imaging modes can detect different types of defects. An idealinspection machine would combine several different modes into a singlesystem. This could reduce inspection costs as well as provideperformance advantages. However, this is very difficult to do inpractice because different imaging modes have different optical design,software, and system requirements. In general, systems that attempt tocombine bright-field and dark-field imaging do not perform either modeas well as single-mode designs.

Bright-field imaging is commonly used in lamp based microscope systems.The advantage of bright-field imaging is the imaged features aretypically readily distinguishable. Image feature size accuratelyrepresents the size of object features multiplied by the magnificationof the optical system. Dark-field imaging is successfully used to detectfeatures on objects. The advantage of dark-field imaging is that flatspecular areas scatter very little light toward the detector, resultingin a dark image. Any surface anomalies or features protruding above theobject scatter light toward the detector. Thus, in inspecting objectssuch as semiconductor wafers, dark-field imaging produces an image offeatures, particles, or other irregularities on a dark background.Normal incidence laser dark-field is a flexible imaging mode that issuited for detecting microscratches.

Examples of inspection systems include high numerical aperture (NA)catadioptric systems with central obscurations, such as those shown inU.S. Pat. No. 5,717,518 by Shafer et al., and U.S. Pat. No. 6,064,517 byChuang et al. Bright field and dark field imaging is employed in, forexample, U.S. Pat. No. 6,288,780 by Fairley et al., U.S. Pat. No.5,153,668 by Katzir, and U.S. Pat. No. 5,058,982 by Katzir et al.Fourier filtering in laser dark field imaging has been employed incertain designs, such as those shown in U.S. Pat. No. 5,177,559 byBatchelder et al. and U.S. Pat. No. 5,428,442 by Un et al.

Certain of the previous designs exhibit beam delivery issues whenperforming dark-field imaging. For example, high NA systems can havedark-field illumination capabilities limited by the central obscurationin the catadioptric optical design. In these systems dark-fieldillumination can be limited to angles greater than a certain value, suchas five degrees. Off axis dark-field imaging and low NA bright-fieldimaging has been employed to address this angular illumination issue incatadioptric designs. In such a design, the dark-field illumination maybe a high NA annular illumination scheme where bright-field illuminationand imaging may be limited to those NAs that can pass through a hole inthe center of the focusing mirror element. Such a construction can alsolimit dark-field illumination and collection angles. Another priorapparatus for combining bright-field and dark-field into one design usesoff axis dark-field and low NA bright-field imaging. The dark-fieldillumination is a high NA linear type of illumination. The bright-fieldillumination and imaging is again limited to those NAs that can passthrough a slot in the center of the focusing mirror elements, againlimiting dark-field illumination and collection angles.

One prior method for achieving Fourier filtering with laser dark-fieldimaging uses a collimated beam of monochromatic light to illuminate asemiconductor wafer from outside the objective between an angle of 82degrees from the normal and the NA defined by the imaging objective.Before forming a dark field image, the collected light passes through aFourier filter to attenuate the spatial frequency componentscorresponding to repeating array patterns. This laser directionaldark-field method illuminates the wafer outside the NA of the imagingobjective. For this reason, the illumination angles can be limited tobetween 82 degrees from the normal and the NA defined by the imagingobjective. Collection angles are also limited to the range of angleswithin the NA of the objective. A long working distance objective isnecessary to allow access by the laser to the area of interest on thesemiconductor wafer. Objectives used in dark field applications of thistype are generally limited to NAs less than 0.7, which corresponding tocollection angles of only up to 44 degrees from normal. A major drawbackof this approach is the Fourier distribution collected by the optics ishighly directional, where only higher angles are collected from one sideof the scattered and diffracted light distribution.

Another prior method for achieving Fourier filtering with laserdark-field imaging uses a collimated beam of monochromatic lightilluminating the wafer from inside the optical system within the NAdefined by the objective. If the system encounters a specific range ofdefect sizes, the illumination angle on the wafer is chosen so theoptical system collects those spatial frequencies of interest. This is alaser directional dark-field method wherein the laser illuminates thewafer from inside the NA as defined by the objective. The problem withthis technique is that small amounts of scattered and reflected lightfrom lens elements in this design have the ability to produce noise atlevels that compromise sensitivity. Introducing laser illumination nearthe pupil location in the imaging system can cause a significant amountof back-scattered and reflected light from the multiple lens surfacestraversed by the illuminating light. The system must also deal withforward-scattered light from the specularly reflected component from thewafer, a significant potential problem. Another problem with thistechnique is that the system uses the same objective pupil plane forinjecting the illumination and processing the light collected by theobjective. This objective pupil feature limits the usable types ofillumination and Fourier filtering. Achieving a higher NA is alsodifficult using this design. Higher NA lenses generally require moreoptical elements, further increasing scattering noise.

In general, it can be difficult to offer both bright-field anddark-field imaging in a high NA inspection system while offering Fourierfiltering, operation over a desirable range of wavelengths, anddark-field illumination over a desirable range of angles.

It would therefore be beneficial to provide a system that delivers abeam of laser dark-field illumination that overcomes the foregoingdrawbacks present in previously known imaging systems. Further, it wouldbe beneficial to provide an optical inspection system design havingimproved functionality over devices exhibiting the negative aspectsdescribed herein.

SUMMARY OF THE INVENTION

According to one aspect of the present design, there is provided anapparatus for inspecting a specimen. The apparatus comprises a primaryillumination source, a catadioptric objective exhibiting a centralobscuration that accepts light energy received from the primaryillumination source and delivers light energy at angles substantiallygreater than normal toward the specimen. Substantially greater thannormal represents angles substantially greater than approximately zerodegrees. The apparatus further comprises an optical device positionedwithin the central obscuration resulting from the catadioptric objectivefor receiving further illumination from a secondary illumination sourceand diverting the further illumination at near normal incidence to thespecimen.

According to another aspect of the present design, there is provided amethod for inspecting a specimen. The method comprises illuminating asurface of the specimen at a variety of angles using a primaryillumination source, illuminating the surface using a secondaryillumination source, the illuminating by the secondary illuminationsource occurring at a substantially normal angle of incidence; andimaging all reflected, scattered, and diffracted light energy receivedfrom the surface onto a detector.

These and other advantages of the present invention will become apparentto those skilled in the art from the following detailed description ofthe invention and the accompanying drawings.

DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which:

FIG. 1 is a prior art broad-band catadioptric imaging system suitablefor use in the present design;

FIG. 2 is a prior art narrow-band alternate catadioptric imaging systemalso suitable for use in the present design;

FIG. 3 illustrates ray paths traced through a catadioptric lens withcentral obscuration, where the central region free of illumination raysis not used for imaging;

FIG. 4 represents a Broad-band catadioptric imaging system withlamp-based bright field illumination and autofocus system in accordancewith the present design;

FIG. 5 is a catadioptric imaging system with normal-incidence laserillumination, beam transport optics, and collimator in accordance withthe present design;

FIG. 6 is a detailed view of a field aperture placed at a field plane tolimit illumination light;

FIG. 7 represents a prism beam turning configuration located within theshadow of the central obscuration;

FIG. 8 shows a suspended mirror beam turning configuration locatedwithin the shadow of the central obscuration;

FIG. 9 is a suspended mirror beam turning configuration mounted on atransparent plate added to the imaging path;

FIG. 10A presents a Fourier filter transmitting a fraction of eachquadrant;

FIG. 10B is a further Fourier filter transmitting a different fractionof each quadrant; and

FIG. 10C is an additional Fourier filter transmitting a specific zone ofnumerical aperture from the available range.

DETAILED DESCRIPTION OF THE INVENTION

The present design is an apparatus that can support both broad-bandbright-field inspection and laser dark-field inspection techniqueswithout compromising the performance of either technique. To accomplishthis enhanced dual-mode inspection capability, both inspectiontechniques use the same imaging system, detection system, and autofocussystem. The illumination systems employed differ from those previouslyavailable, and a Fourier filter or other pupil aperture is employed fornormal incidence laser dark-field modes. A detector with a suitabledynamic range may be employed in the detection system to handle the highdynamic range signals produced by the normal incidence dark-fieldinspection technique.

The present design is useful for beam delivery for illuminating surfacesfor optical inspection with laser sources. The design is particularlysuited for use with optical systems that have a central obscurationwhich blocks light from traversing along the optical axis of the imagingsystem. One embodiment of the invention allows the laser light to enterinto the objective using, in part, components located substantiallywithin the central obscuration region. This light is then redirectedalong the optical axis of the objective. This normal incidenceillumination technique results in dark-field imaging when the componentsare aligned such that the specular reflection from the inspected surfaceis reflected back along the illumination path and is not collected bythe imaging system. Both the illuminating light and the specularlyreflected light pass only a few objective lens surfaces so thatscattering noise from the optics is reduced. Other embodiments allowoff-axis illumination in a similar manor.

The normal-incidence dark-field illumination path may use a relativelysmall mirror or prism component selectively positioned to direct thelight entering the imaging system from the side to a path along theoptical axis and normal to the surface. The mirror or prism componentmay be mounted directly onto an existing optical element in the system,mounted onto the central obscuration component, or mounted on a supportstructure that is also substantially within the obscuration region. Ifthe component is made to fill the central obscuration, then unwantedreflection and scattering can be further reduced. Off axis illuminationis also possible using a similar design.

The components of the beam delivery system for laser dark-fieldillumination do not impact other forms of illumination or imaging suchas broad-band bright-field illumination and imaging. Thereforedark-field illumination and inspection may function independently ofbright-field illumination. The configuration also allows forsimultaneous operation of different illumination and imaging techniques.For example, conventional broad-band bright-field using lamp basedillumination may operate simultaneously with laser illumination. Thefinal image may then contain both light sources. The balance of lightbetween these sources can be controlled to optimize the inspectionperformance. Alternatively, the final image may be separated usingwavelength-selective elements into, for example, a bright field lampimage and a dark field laser image. Separate optical or electronicprocessing and detection can then be employed for each image.

The present designs are well suited for use with Fourier filteringtechniques. In addition, laser Fourier filtering techniques can be usedsimultaneously with broad-band bright-field imaging.

One embodiment of an imaging system suitable for use with thisembodiment is shown in FIG. 1. The design of FIG. 1 is a catadioptricimaging system described in U.S. Pat. No. 5,999,310, entitled“ULTRA-BROAD-BAND UV MICROSCOPE IMAGING SYSTEM WITH WIDE RANGE ZOOMCAPABILITY,” filed on Aug. 7, 1997, which is a continuation in part ofU.S. Pat. No. 5,717,518, entitled “BROAD SPECTRUM ULTRA VIOLETCATADIOPTRIC IMAGING SYSTEM” filed on Jul. 22, 1996, both of which areassigned to KLA-Tencor Corporation and both of which are herebyincorporated by reference.

The design of FIG. 1, similar to the design of FIG. 4 of the '310patent, comprises a catadioptric objective section 128 and a zoomingtube lens group sections 139. The catadioptric objective section 128comprises a catadioptric lens group 122, a field lens group 127, and afocusing lens group 129. The system images an object (not shown), suchas a semiconductor wafer, to the image plane or detector array 140. Thecomplete 0.9 NA catadioptric objective section 128 is also described inthe '518 patent.

The catadioptric objective section 128 is optimized for ultra-broadbandimaging in the UV spectral region (about 0.20 to 0.40 micronwavelength). The design uses the Schupmann principle in combination withan Offner field lens to correct for axial color and first order lateralcolor, and an achromatized field lens group to correct the higher orderlateral color. The catadioptric lens group 122 includes a near planar orplanar reflector 123, which is a reflectively coated lens element, ameniscus lens 125, and a concave spherical reflector. This design uses aconcave reflector 124 and a large meniscus lens 125 to simplifymanufacturing. Both reflective elements have central optical apertureswithout reflective material to allow light from the intermediate imageplane 126 to pass through the concave reflector, be reflected by thenear planar (or planar) reflector 123 onto the concave reflector 124,and pass back through the near planar (or planar) reflector 123,traversing the associated lens element or elements on the way.

The achromatic multi-element field lens group 127 may be made from twoor more different refractive materials, such as fused silica andfluoride glass, or diffractive surfaces. The field lens group 127 may beoptically coupled together or alternatively may be spaced slightly apartin air. Because fused silica and fluoride glass do not differsubstantially in dispersion in the deep ultraviolet range, theindividual powers of the several component element of the field lensgroup need to be of relatively high magnitude. Use of such an achromaticfield lens allows the complete correction of axial color and lateralcolor over an ultra-broad spectral range. In the simplest version of thedesign, only one field lens component need be of a refractive materialdifferent than the other lenses of the system. The field lens group 127is moved slightly from the intermediate image location to decrease theheat load and surface scattering of the field lens group 127.

The design of FIG. 1 has a focusing lens group 129 with multiple lenselements, preferably all formed from a single type of material, withrefractive surfaces having curvatures and positions selected to correctboth monochromatic aberrations and chromatic variation of aberrationsand focus light to an intermediate image. In the focusing lens group 129a special combination of lenses 130 with low power corrects the systemfor chromatic variation in spherical aberration, coma, and astigmatism.

Design features of the field lens group 127 and the low power group 130provide significant optical advantages. The zooming tube lens system 139combined with the catadioptric objective 128 can allow the detectorarray 140 to be stationary during zooming, although the design is notlimited to the components and orientation shown. For example, assumingthat the catadioptric objective system 128 does not also have anyzooming function, two general design possibilities may be available forzooming tube lens system or zooming section 139.

First, the zooming section 139 can be all the same refractive material,such as fused silica, and can be designed so that primary longitudinalcolor and primary lateral color do not change during zooming. Primarychromatic aberrations do not have to be corrected to zero, and typicallycannot be corrected to zero if only one glass type is used, but theygenerally have to be stationary. Then the design of the catadioptricobjective 128 can be modified to compensate for these uncorrected butstationary chromatic aberrations of the zooming tube lens. The combinedmicroscope system shown in FIG. 1 may be constructed from a singlematerial, i.e., fused silica, except for the calcium fluoride or adiffractive surface in the achromatized Offner-type field lens.

Second, the zooming tube lens group 139 can be corrected for aberrationsindependently of the catadioptric objective 128. Such a construction canrequire at least two refractive materials with different dispersions,such as fused silica and calcium fluoride, or diffractive surfaces. Theresult may be a tube lens system that, because of unavoidablehigher-order residuals of longitudinal and lateral color over the entirezoom range, is generally not capable of high performance over a verybroad UV spectral region. Compromises can be made in reducing thespectral range, the numerical aperture, the field size of the combinedsystem, or some combination of these compromises.

The design of FIG. 1 straddles the two situations just described. Thezooming tube lens 139 may first be corrected independently of thecatadioptric objective 128, using two refractive materials (such asfused silica and calcium fluoride). Zooming tube lens 139 may then becombined with the catadioptric objective 128. The catadioptric objectivecan be modified to compensate for the residual higher-order chromaticaberrations of the zooming tube lens system, an effect achievable due tothe design of the field lens group 127 and the low power lens group 130of the catadioptric objective. The combined system can then be optimizedwith all parameters being varied to achieve the best performance.

A tube lens section can be designed such that its higher-order chromaticaberrations do not change by any significant amount during zoom. If thedetector array 140 is allowed to move during zoom, then the designproblem becomes much easier, but that is not nearly as desirable ashaving an image position fixed relative to the rest of the system.

The imaging system of FIG. 1 is corrected for wavelengths from 230-370nm, enabling the system to support both broad-band bright-field imagingas well as normal incidence laser dark-field imaging at any wavelengthwithin this range. An optical system with a narrower correction range isalso possible, but this can limit the lamp bright-field and laserdark-field illumination and imaging wavelengths. The imaging system usesa single moving tube lens group to change the magnification from 38× to200×. Magnification may be changed by moving the tube lens group andadjusting the optical path to the detector to optimize performance.

An alternate arrangement comprises an apparatus that supportsnarrow-band bright-field inspection and laser dark-field inspectiontechniques without compromising the performance of either technique.This system combines laser bright field illumination and normal laserdark field illumination. The bright field laser illumination system mayincorporate elements to reduce the laser's apparent temporal and spatialcoherence through use of techniques such as a moving optical diffuserelement. An imaging system suitable for use with this arrangement isshown in FIG. 2. The design of FIG. 2 contains a catadioptric imagingsystem 200 optimized for a single wavelength. This type of catadioptricimaging system is extensively discussed in U.S. Pat. No. 6,064,517,titled “HIGH NA SYSTEM FOR MULTIPLE MODE IMAGING,” inventors Yung-HoChuang et al., issued May 16, 2000, the entirety of which is herebyincorporated by reference.

The prescriptions for the optical components of FIG. 2 are as shown inTable 1. TABLE 1 Prescriptions for optical components in FIG. 2 SurfRadius Thickness Material OBJ Infinity 149.014 1 27.587 10.534 Fusedsilica 2 25.780 5.071 3 −20.905 3.125 Fused silica 4 132.384 2.282 5−136.513 17.944 Fused silica 6 −34.990 1.010 7 126.993 8.000 Fusedsilica 8 −58.025 423.599 9 −1694.877 10.000 Fused silica 10 −127.963192.592 11 140.480 7.000 Fused silica 12 1002.314 152.910 STO Infinity82.734 14 72.803 7.500 Fused silica 15 −72.803 2.586 16 54.855 5.500Fused silica 17 Infinity 13.747 18 −48.455 3.000 Fused silica 19−148.848 46.133 20 −13.322 4.416 Fused silica 21 −15.209 107.298 22121.792 10.000 Fused silica 23 46.578 5.083 24 292.207 6.500 Fusedsilica 25 −87.559 0.100 26 132.564 4.000 Fused silica 27 39.354 5.100 28182.579 7.000 Fused silica 29 −113.665 0.100 30 28.442 15.931 Fusedsilica 31 151.603 6.997 32 −70.100 10.772 Fused silica 33 33.922 16.56934 270.750 24.896 Fused silica 35 −34.371 6.205 36 197.516 5.500 Fusedsilica 37 −197.516 0.100 38 98.238 5.500 Fused silica 39 −446.331 0.10040 52.767 5.000 Fused silica 41 319.544 15.000 42 Infinity 52.552 43Infinity 23.328 Fused silica 44 Infinity −23.328 MIRROR 45 Infinity−52.552 46 87.869 52.552 MIRROR 47 Infinity 23.328 Fused silica 48Infinity 0.750 IMA Infinity

From the foregoing table, as with all other tables including lensprescriptions used herein, “Surf” or similar designation represents thesurface number associated with a particular element. Most elements havetwo surfaces, and thus correlating FIG. 2 with the foregoing table,surface OBJ has an infinite radius of curvature and represents thetopmost element in FIG. 2. Surface 1 is the topmost lens surface in thedrawing and is 10.534 millimeters from surface OBJ and has a radius ofcurvature of 27.587 millimeters. The center of surface 2 is 5.071millimeters from the center of surface 1, and the lens has a thicknessof 5.071 millimeters and the lens is formed of fused silica. In thismanner, dimensions of every surface in the figures presented can beascertained.

The broad-band catadioptric imaging system of FIG. 2 is corrected atnumerical apertures of up to 0.9 corresponding to illumination andcollection angles of up to 64 degrees. The design of FIG. 2 includesvarious elements broken into groups for the purpose of discussion.Catadioptric group 201 includes mirror element 205 and catadioptricelement 206. Field/focusing lens group 202 comprises a plurality ofprecision lenses. A detector may be positioned at point 204. Precisionlensing group 203 includes several additional lenses useful in enablingthe high NA and beneficial aspects discussed in the '517 patent infurther detail. The catadioptric imaging system 200 has a centralobscuration due to the holes in, for example, catadioptric element 205and flat element 206. In this construction, the optical design has anobscuration of approximately 10 percent of the pupil diameter or 1percent of the pupil area. Manufacturing tolerances can increase thisobscuration to approximately 12 percent of the pupil diameter orapproximately 1.4 percent of the pupil area. This obscurationcorresponds to a NA of 0.11. Thus, the imaging NA in the design of FIG.2 is variable from 0.11 to the full 0.9 by using an aperture at thepupil plane.

Regarding “pupil plane,” a pupil in an optical system is an image of theaperture stop. At least two pupils and thus two pupil planes are presetin an optical system, one in object space called the entrance pupil orentrance pupil plane and one in image space called the exit pupil orexit pupil plane. As used herein, the term “pupil plane” may mean eitherthe entrance or exit pupil plane depending on the context.

The effect of obscuration on the light path in the objective is shown infurther detail in FIG. 3, illustrating field/focusing lens group 301,catadioptric group 302, and field point 303. The catadioptric group 301here comprises reflector 305, curved lens 306, and flat lens/mirrorelement 307.

The addition of broad-band bright-field illumination and autofocus tothe catadioptric imaging system is shown in FIG. 4. Bright-fieldillumination in this illustration includes a Hg—Xe arc lamp 401, butother light sources may be employed, collection optics 402, cold mirror403, homogenizer 404, relay optics 405, and an autofocus beam splitter406. These devices are arranged in a standard microscope illuminationscheme. Light may be redirected using beamsplitter/reflective surface450 to introduce bright-field illumination light into the catadioptricimaging system. FIG. 4 further illustrates catadioptric group 411,focusing lens group 412, and precision lensing group 413, and presentsan image plane at point 420, where again a sensor may be positioned.FIG. 4 includes optical components that are identical to those shown inFIG. 1. The prescriptions for the optical components of FIG. 4,specifically those in the primary line of illumination, are as shown inTable 2. TABLE 2 Prescriptions for optical components in FIGS. 1, 4, and5 Surf Radius Thickness Material OBJ Infinity 1.000 1 Infinity 1.000Fused Silica 2 Infinity 120.000 3 40.973 9.762 Fused Silica 4 46.97916.920 5 −30.067 3.500 Fused Silica 6 86.035 3.000 7 −71.814 5.500 FusedSilica 8 −25.118 0.500 9 −127.725 4.000 Calcium Fluoride 10 37.60213.711 11 54.642 9.000 Calcium Fluoride 12 −30.067 0.100 13 −30.2904.500 Fused Silica 14 −141.249 191.535 15 137.264 4.500 Fused Silica 1658.294 2.750 17 275.840 6.000 Fused Silica 18 −87.851 76.110 19 30.7266.000 Fused Silica 20 19.670 12.102 21 −25.298 7.455 Fused Silica 22−48.347 1.000 23 79.720 7.000 Fused Silica 24 −93.732 4.000 STO 24.23412.000 Fused Silica 26 −453.150 4.121 27 −50.118 10.000 Fused Silica 28−127.725 14.254 29 42.780 10.000 Fused Silica 30 −33.255 0.960 31−20.242 3.000 Fused Silica 32 50.846 0.100 33 13.926 6.607 CalciumFluoride 34 −34.229 0.100 35 33.741 2.000 Fused Silica 36 3.813 0.020 373.813 3.000 Calcium Fluoride 38 70.287 7.000 39 Infinity 10.023 4099.998 11.000 Fused Silica 41 80.583 29.230 42 1021.400 10.000 FusedSilica 43 Infinity −10.000 MIRROR 44 1021.400 −29.230 45 80.583 −11.000Fused Silica 46 99.998 −10.023 47 74.989 10.023 MIRROR 48 99.998 11.000Fused Silica 49 80.583 29.230 50 1021.400 10.000 Fused Silica 51Infinity 1.500 IMA Infinity

Various autofocus systems can be used to control the focus position ofthe specimen or sample surface. The autofocus mechanism 475 isillustrated in FIG. 4. One autofocus technique is described in U.S. Pat.No. 4,639,587 entitled “AUTOMATIC FOCUSING SYSTEM FOR A MICROSCOPE”filed on Jan. 22, 1985 which is hereby incorporate by reference. Thistype of autofocus operates at a different wavelength than theillumination and can be introduced using a dichroic mirror, shown as thereflective surface 450 in FIG. 4. This mirror can be placed in theillumination system.

The addition of a normal incidence laser dark-field illumination to thecatadioptric imaging system is shown in FIG. 5. The dark-fieldillumination includes a UV laser 501, adaptation optics 502, an apertureand window 555 in the mechanical housing 551, and a prism 503 toredirect the laser along the optical axis at normal incidence to thesurface. The prism 503 also directs the specular reflection fromspecimen surface features and reflections from the optical surfaces outof the objective 550 along the path of the illumination laser. Again,lensing is provided in the general form of a catadioptric group 552,focusing/field group 553, and precision lensing group 554.

Many different laser sources are available within the 230-370 nmwavelength band supported by the catadioptric imaging system 500. Forexample, continuous wave (CW) or pulsed lasers at wavelengths of 363.8,354, 351, 333, 325, 275, 266, 257, 248, 244 nm, and so forth, can beused. For many applications a CW laser may be useful for inspection. ACW laser can eliminate timing problems between the laser and dataacquisition. Adaptation optics can be used to control the illuminationbeam size and profile on the surface being inspected, and the prismelement may also provide illumination control. An optional zoom systemcan change the illumination beam size to match the field size whenimaging magnifications change. Such a zoom system can minimize the laserenergy required to inspect at small field sizes.

A field aperture 600 as shown in FIG. 6 may also be used to limitunwanted scattering light from entering the system. The field aperturecan optionally be imaged to the surface being inspected using additionaloptics. The polarization of the illumination can be chosen to optimizethe signal-to-noise ratio for defects of interest. Both linear andcircular polarization may be employed. For example, in semiconductorwafer inspection, linear polarization oriented at 45 degrees to theperiodic structures on the semiconductor wafer can improve defectdetection by reducing diffraction noise from these structures. After thelight beam passes the adaptation optics, the light beam can enter theobjective through a hole in the side of the mechanical assembly. Awindow (not shown) may be used to cover this hole and keep contaminationfrom entering the objective. A window can also prevent any nitrogenpurge gas from escaping. Once the laser beam enters the objective, lightenergy reflects off a prism 701 mounted in the region of the centralobscuration 710 of the imaging system 400 as shown in FIG. 7. Prism 701has a top surface 702 and a bottom surface 703. In the arrangementpresented the prism 701 does not interfere with broad-band bright-fieldillumination or imaging. The prism reflects laser light energy andredirects the light energy along the optical axis of the imaging systemat an angle normal to the surface 704. The prism 701 can be mounted tothe plate 705 using optical contacting, epoxy or other adhesive,drilling a small recess in the plate, or other mechanical mountingprocedure known to those skilled in the art. Mounting the prism to theplate 705 tends to reduce any ghost reflections from the top surface ofthe plate 705. The internal angle of the prism can be chosen so thelight is totally internally reflected from the second surface 703,tending to eliminate the need for a mirror coating.

A mirror may also be employed to direct the beam along the optical axisnormal to the specimen or wafer. In an alternative embodiment, themirror or prism 801 is attached to the meniscus lens 802 via a connector803 as shown in FIG. 8. In yet another embodiment, the optical device901, such as a prism or mirror, is suspended below the plate 905 in thecentral obscuration 910 by a “spider” 902 as shown in FIG. 9. The spider902 generally enables firmly suspending the prism or mirror 901 belowthe plate and may include one, two or has three or more thin supportsthat have a minimal effect on the bright-field illumination and imaging.As shown in FIG. 9, dark-field light energy passes throughwindow/aperture 950, strikes optical device 901 and is directed towardthe surface 975 of the specimen or sample.

In normal incidence laser dark-field, limiting specular reflections,ghost reflections, and unwanted scattering light from the detectorprovide certain benefits. These effects are beneficial because thescattered and diffracted dark-field signal is generally much weaker thanthese noise sources. A prism delivery system such as that shown hereeliminates the specular reflection from the surface as well as the ghostreflections because such reflections exit along the path of theilluminating beam. The primary noise source is from the scattering ofthe bottom surface of the plate 705. This scattering is relatively smalland has no direct path to the sensor and thus presents minimal problems.

The scattering and diffracted light from the surface features iscollected by the imaging system and mapped to the pupil plane. Eachlocation in the pupil plane represents light collected at differentscattering angles above the surface. Apertures can be placed at thepupil plane to select only the scattering angles of interest and improvethe signal-to-noise ratio for the defect of interest. For example, theapertures shown in FIGS. 10A and 10B can limit scattered and diffractedlight reaching the detector to the four quadrants of the pupil.

The apertures of FIGS. 10A and 10B could be employed in semiconductorinspection. Light diffracted from the patterns on the wafer or reticlesurface is primarily diffracted in the X and Y directions. Thisdiffracted light is blocked by the aperture, such as aperture 1001 or1002, allowing scattered light from any defects on the pattern to passthrough to the detector. The aperture 1003 in FIG. 10C can limit thescattered and diffracted light reaching the detector to the centralregion of the pupil. Limiting light reaching the detector (not shown)can be useful for detecting microscratches on a semiconductor wafer.

Normal incidence laser dark field imaging has the advantage that Fourierfiltering can be used to minimize repeating pattern signals on thesurface and enhance the defect signal-to-noise ratio. Normal incidencelaser dark-field generally provides a suitable Fourier filter design,known in the art and generally described in U.S. Pat. No. 6,020,957, “ASYSTEM AND METHOD FOR INSPECTING SEMICONDUCTOR WAFERS,” inventorsEliezer Rosengaus and Steven R. Lange. The '957 patent is assigned toKLA-Tencor Corporation and is incorporated herein by reference.

A Time Delay and Integration (TDI) sensor or detector, not shown but maybe positioned, for example, at point 420 in FIG. 4, may be used todetect the broad-band bright-field and normal incidence laser dark-fieldsignals. Use in such a system can require a sensor with high sensitivityfor the bright-field signal and high dynamic range for the dark-fieldsignal. The bright-field and dark-field signals may be detectedseparately or combined together on the sensor. Alternatively, thebright-field and dark-field signals may be separated and sent toseparate TDI sensors.

The present system design may be employed in various environments,including but not limited to lithography, microscopy, biologicalinspection, medical research, and the like.

The design presented herein and the specific aspects illustrated aremeant not to be limiting, but may include alternate components whilestill incorporating the teachings and benefits of the invention, namelya beam delivery system for laser dark-field illumination in acatadioptric optical system. While the invention has thus been describedin connection with specific embodiments thereof, it will be understoodthat the invention is capable of further modifications. This applicationis intended to cover any variations, uses or adaptations of theinvention following, in general, the principles of the invention, andincluding such departures from the present disclosure as come withinknown and customary practice within the art to which the inventionpertains.

1. An apparatus for inspecting a specimen, comprising: a primaryillumination source; a catadioptric objective exhibiting a centralobscuration that accepts light energy received from the primaryillumination source and delivers light energy at angles substantiallygreater than normal, wherein substantially greater than normalrepresents angles substantially greater than approximately zero degrees,toward the specimen; and an optical device positioned within the centralobscuration resulting from the catadioptric objective for receivingfurther illumination from a secondary illumination source and divertingthe further illumination at near normal incidence to the specimen. 2.The apparatus of claim 1, further comprising a detector arrangementsensitive to a wavelength of illumination from the secondaryillumination source.
 3. The apparatus of claim 1 where the centralobscuration is less than approximately 15% of a pupil diameter of thecatadioptric objective.
 4. The apparatus of claim 1 where the opticaldevice comprises a prism.
 5. The apparatus of claim 1 where the opticaldevice comprises a reflective surface.
 6. The apparatus of claim 2 wherethe detector arrangement comprises a plurality of detectors.
 7. Theapparatus of claim 1 where the primary illumination source comprises anarc lamp.
 8. The apparatus of claim 1 where the secondary illuminationsource wavelength comprises at least one wavelength from a groupcomprising 193, 198, 213, 244, 257, 266, 363.8, 354, 351, 355, 325, and532 nm.
 9. The apparatus of claim 2 wherein the detector arrangementcomprises at least one Time Delay and Integration sensor.
 10. Theapparatus of claim 1, further comprising an optical Fourier filteraperture located at a pupil plane of the apparatus, and wherein theoptical Fourier filter aperture attenuates repeating pattern signalsreceived from the specimen.
 11. The apparatus of claim 10 where theoptical Fourier filter aperture comprises a spectrally narrow-banddielectric thin film pattern, wherein the spectrally narrow-banddielectric thin film pattern blocks components of the furtherillumination from the secondary illumination source while substantiallytransmitting the illumination from the primary illumination source. 12.An apparatus located within a central obscuration of an opticalinspection system comprising an objective, a primary illumination sourceand a secondary illumination source, wherein the primary illuminationsource provides illumination through the objective and at an anglerelatively larger than normal to a surface of a specimen, and furtherwherein the secondary illumination source provides further illuminationsubstantially normal to the surface of the specimen, the apparatuscomprising: a surface positioned within the central obscuration todirect the further illumination to the surface of the specimen at normalincidence to the surface; and a fastening mechanism positionedpredominantly within the obscuration, the fastening mechanism employedto fixedly position the surface during inspection.
 13. The apparatus ofclaim 12 where the surface and fastening mechanism allow a wavelengthbetween approximately 193 nm and 400 nm to be transmitted to thesurface.
 14. The apparatus of claim 12 where the surface comprises areflective surface.
 15. The apparatus of claim 12 where the surfaceforms part of a prism.
 16. The apparatus of claim 12 where the surfaceis a prism formed within a mechanical support component.
 17. Theapparatus of claim 12 where the fastening mechanism comprises an epoxy.18. The apparatus of claim 12 where the fastening mechanism comprisesoptical contacting.
 19. A method of inspecting a specimen, comprising:illuminating a surface of the specimen at a variety of angles using aprimary illumination source; illuminating the surface using a secondaryillumination source, said illuminating by the secondary illuminationsource occurring at a substantially normal angle of incidence; andimaging all reflected, scattered, and diffracted light energy receivedfrom the surface onto a detector.
 20. The method of claim 19, whereinthe primary illumination source is broad-band and the secondaryillumination source is narrow-band.
 21. The method of claim 19 whereinilluminating the surface using the secondary illumination sourcecomprises directing light energy from the secondary illumination sourceto the surface using a light deflecting device positioned within acentral obscuration of light energy from the primary illuminatingsource.
 22. The method of claim 19, wherein the illuminating by theprimary illumination source occurs contemporaneously with theilluminating by the secondary illumination source.
 23. The method ofclaim 19, wherein the illuminating by the primary illumination sourceoccurs at a time different from the illuminating by the secondaryillumination source.